Death Finds a Way: A Janie Riley Mystery by Lorine McGinnis Schulze. Janie Riley is an avid genealogist with a habit of stumbling on to dead bodies. She and her husband head to Salt Lake City Utah to research Janie's elusive 4th great-grandmother. But her search into the past leads her to a dark secret.
A: The most abundant isotope of magnesium has 12 neutrons; it is therefore, technically known as magnesium-24. Calculating the number of neutrons in an isotope of any element involves subtracting that element's atomic number from its atomic weight. Magnesium's number is 12, and its weight is just over 24.
The number of protons an atom has in its nucleus is given by the atomic number. A quick looks in the periodic table will show that aluminium has an atomic number equal to #13#. This means that any atom that is an isotope of aluminium will have #13# protons in its nucleus. Since you're dealing with a neutral atom, the number of electrons that surround the nucleus must be equal to the number of protons found in the nucleus. Therefore, the aluminium-27 isotope will have #13# electrons surrounding its nucleus. Finally, use the known mass number to determine how many neutrons you have.